A directional antenna may be used to receive a radio wave from a particular direction better than waves from other directions. A Yagi antenna is well-known as a directional antenna. A variable directivity antenna is used to selectively receive a desired one of radio waves from various directions. An example of variable directivity antenna is disclosed in Japanese Utility Model Publication No. SHO 63-38574 Y2 published on Oct. 12, 1988.
The variable directivity antenna disclosed in this Japanese UM publication includes first and second antennas which lie to orthogonally intersect with each other in the same horizontal plane. Dipole antennas or folded dipole antennas are used as the first and second antennas. A signal received by the first antenna is applied through a first variable attenuator to a combiner, and a signal received by the second antenna is applied through a second variable attenuator to the combiner. The directivity of the variable directivity antenna can be varied by adjusting the amounts of attenuation provided by the first and second variable attenuators.
A Yagi antenna can receive better a radio wave from a fixed, particular direction, but it cannot receive well radio waves from other directions. The above-described variable directivity antenna has directivity that can rotate, and, therefore, it can receive only a radio wave from a desired direction selected from radio waves from various directions. However, the variable directivity antenna of Japanese Utility Model Publication No. SHO 63-38574 Y2 has a “8”-shaped directivity pattern, and, therefore, the antenna receives also a radio wave from the direction opposite to the desired direction. In other words, the antenna of Japanese Utility Model Publication No. SHO 63-38574 Y2 has a low F/B ratio.
An object of the present invention is to provide a small-sized antenna that has an improved F/B ratio and can selectively receive well radio waves from different two directions. Another object of the present invention is to provide an antenna system that can selectively receive well desired ones of radio waves from various directions, by the use of the variable directivity antennas.